1. Field of the Invention
The present invention generally relates to a filter and a duplexer, and more particularly, to a filter and a duplexer that are mounted on a mounting unit.
2. Description of the Related Art
As mobile communication systems have been developed dramatically in recent years, portable telephone devices, portable information terminals, and the likes have rapidly spread, and such terminals with smaller sizes and higher performances are being developed. Also, analog systems and digital systems are both used in portable telephone systems, and the frequencies mostly used are in the 800 MHz to 1 GHz band and the 1.5 GHz to 2.0 GHz band. Antenna duplexers utilizing surface acoustic wave filters or piezoelectric thin-film filters have also been developed for the use in those devices designed for mobile communications.
In the recent development of portable telephone devices, the performances of the terminals have been improved by employing the dual mode (a combination of an analog system and a digital system or a combination of the digital TDMA (time division multiple access) and CDMA (code division modulation access)), or the dual band (a combined use of the 800 MHz band and the 1.9 GHz band, or the 900 MHz band and the 1.8 GHz band or the 1.5 GHz band), as a variety of systems have been developed. The components (filters) to be used in those terminals are expected to have more sophisticated functions.
As well as sophisticated functions, smaller and less expensive devices are expected. Many antenna duplexers to be used in multi-functional terminals are formed with dielectrics, with complex duplexers utilizing surface acoustic waves and including dielectrics at least on one side, or only with surface acoustic wave devices.
A dielectric duplexer is large in size, and with a dielectric duplexer, it is very difficult to produce a small-sized or thin portable terminal. Also, where a surface acoustic wave device is used on one side, it is difficult to produce a small-sized or thin device, due to the size of the dielectric device. Among duplexer devices using conventional surface acoustic wave filters, there are devices of a module type that have a transmission filter, a reception filter, and a matching circuit mounted on a printed board independently of one another, and devices of an integrated type that have transmission and reception filter chips mounted in a multilayer ceramic package and a matching circuit mounted in a package. Each of those devices is ⅓ to 1/15 of a dielectric duplexer in volume, and is ½ to ⅓ of a dielectric duplexer in thickness when seen in the height direction. Japanese Unexamined Patent Publication No. 8-18393 discloses a filter and duplexer that have a filter chip mounted on a mounting unit that is a stacked package having ceramic substrates stacked or is a stacked substrate or the like.
Japanese Unexamined Patent Publication Nos. 2004-135322 and 2003-332885 disclose the technique of connecting an inductor in parallel to a series-arm resonator of a ladder filter or a duplexer utilizing the ladder filter. As an inductor is connected in parallel to a series-arm resonator, an attenuation pole can be formed adjacent to the pass band of the filter, and the suppression characteristics can be improved.
Where a filter chip having an inductor connected in parallel to a series-arm resonator is mounted on a mounting unit, and the size is made smaller accordingly, the mutual inductance between signal lines needs to be made lower so as to achieve a higher degree of suppression.